For example, when cutting the material to be cut such as a steel plate or a stainless steel plate, the plasma cutting method capable of improving a cutting speed with respect to the gas cutting method is increasingly adopted. In the plasma cutting method, in order to cut the material to be cut, a plasma arc is sprayed to the material while melting the base material by the heat of the plasma arc and removing the molten material by the energy of the sprayed plasma arc.
The Applicant owns the patent of the plasma torch indicated in Patent Document 1. Further, the Applicant proposed a nozzle capable of realizing the effective cooling by cooling water by attaching it to the plasma torch (see Patent Document 2). The configurations of the plasma torch and the nozzle will be briefly described with reference to FIG. 5.
In the nozzle 52, the ejecting hole 52a for spraying plasma arc is formed in the center. Further, the annular passage 52b of the cooling water and a plurality of independent water passages 52c which communicate with the annular passage 52b are formed on the nozzle 52. By attaching the nozzle 52 with the torch body 51, the water passages 53a and 53b for cooling the nozzle 52 are formed in the plasma torch. In particular, the torch body 51 has the water supply port 54a which is connected to the water passage 53a for supplying cooling water to the nozzle 51 and the water drain port 54b which is connected to the water passage 53b for draining the cooling water which passes through the nozzle 52.
Both of the water supply port 54a and the water drain port 54b or either the water supply port 54a or the water drain port 54b are configured as a groove connected to the water passages 53a and 53b, which extends on a plain which crosses the axes of the water passages 53a and 53b. Further, the interval on the circumference of a circle between the ends of the grooves or the interval on the circumference between an end of one of the grooves and an end of the supply port 54a or the drain port 54b is greater than the width on the circumference of a circle of the independent water passage 52c formed in the nozzle 51 or the pitch on the circumference of the independent water passage. Thus, when the nozzle 52 is attached, one of the independent water passages 52c formed in the nozzle 52 is communicated with the water supply port 54a. Further, another independent water passage 52c is communicated with the drain port 54b. 
A plurality of divided pieces 55c for forming the water passage 52c is formed on the outer periphery of the nozzle 52. The nozzle 52 has the inner nozzle 55 whose end surface 55a on the rear end side (the plasma torch side) is an energizing surface, the outer nozzle 56 for forming the cooling water passage formed between the inner nozzle 55 and the outer nozzle 56, and the secondary airflow cap 57 for fastening the nozzle 52 to the torch body 51.
When screwing the secondary airflow cap 57 in attaching the nozzle 52 to the torch body 51, the nozzle 52 is fixed when the end face 55a on the rear side (the plasma torch side) of the inner nozzle 55 becomes in contact with the energizing surface 58a formed on the nozzle base 58 of the plasma torch. With the fixation of the nozzle 52, the annular cooling water passage 52b is formed on the position closer to the distal end side than the divided pieces (the ejecting hole side) between the inner nozzle 55 and the outer nozzle 56. Further, the plurality of independent cooling water passages 52c which communicate with the cooling water passage 52b are formed by the divided pieces. Thus, one of the independent cooling water passages 52c formed in the nozzle 52 communicates with the water supply port 54a for supplying the cooling water and the cooling water provided to the nozzle 52 flows through the independent water passage 52c to the annular cooling water passage 52b on the distal end side and thereafter the cooling water is discharged. Thus, the nozzle 52 can be cooled till the distal end portion of the nozzle 51 thereby preventing excessive heating due to the heat of the plasma arc.